The present invention relates in general to polarimeters, and more specifically to polarimeters that are used to measure optically active fluid samples.
An existing multiple wavelength polarimeter is shown in FIG. 1. A broadband light source 1 projects a beam of substantially parallel light through fixed polarizer 2. The light beam 21, which after passing through polarizer 2 consists of single-plane polarized components at various wavelengths, enters Faraday cell 3. Faraday cell 3, typically constructed of a transparent rod of suitable material arranged axially to the beam, is wound with a coil carrying an oscillating current transmitted by signal generator 4. The oscillating current signal causes the plane of polarization of the parallel-light beam to oscillate about the fixed direction established by polarizer 2 with an amplitude proportional to the current in the coil. The light beam 21 then passes through the optically active sample 24 contained in sample cell 5 (also referred to herein as container 5). The optical characteristics of the sample 24 generally impart additional rotation of the mean plane of polarization of the light beam 21 relative to the polarization of the beam established by fixed polarizer 2. The additional rotation imparted by the beam 21 passing through the sample may be proportional to the concentration of an optically active constituent in the sample 24 being measured. The term “optically active sample” is intended to include the case of a null or blank sample with an optical activity of zero, in addition to real samples which operate to change one or more characteristics of light beam 21.
Light beam 21 may then pass through analyzer 6. Analyzer 6 may include a polarizer mounted so as to be rotatable about the axis of the beam. The rotational position of the polarizer may be determined by controller 7 acting through the motor and encoder unit 8. The wavelength of interest can then be isolated by wavelength selector 9. Wavelength selector 9 may include a motorized monochromator or filter wheel. The intensity of the beam 21 arriving at the detector 10 is generally proportional to the square of the cosine of the angle between the beam polarization direction upon exit from sample cell 5 and the analyzer polarization direction.
Fourier analysis of the beam intensity variation with time determines the sign of the minimum angle separating (a) the beam polarization direction upon exit from sample cell 5 from the (b) analyzer 6 polarization direction that is needed in order to null the rotation of the sample. If this minimum angle is sufficiently small relative to the amplitude of the oscillating polarization produced by the Faraday cell 3, then the magnitude of the minimum angle can be determined as well.
Together, this sign and magnitude information is used to rotate the analyzer 6 to extinguish or “null” the component of intensity that is due to the rotation of polarization plane induced by the sample to be measured. The analyzer 6 angle needed to null the system in this manner, when no sample is present in sample container 5, becomes the zero reference. Any additional analyzer 6 angle needed to null the system when a sample 24 to be measured is present in container 5 constitutes a measurement of the optical rotation caused by the sample 24. This additional analyzer 6 angle (i.e. the analyzer angle over and above the angle needed to null the system when no sample is present in sample cell 5) may be proportional to the concentration of an optically active constituent in the sample 5 being measured.
In existing polarimeters of the type depicted in FIG. 1, one source of measurement error is the wavelength error of the wavelength selector 9. Commercially available compact monochromators intended for use in benchtop analytical instrumentation typically have a wavelength accuracy on the order of 1 nanometer (nm) and a wavelength repeatability of plus or minus 0.2 nm. If wavelength repeatability errors of this magnitude were present during optical rotation measurements of a normal sucrose solution at a wavelength of 589 nm at 20° C. (degrees Celsius), the contribution of this error source to the optical rotation repeatability would be about 0.025 degrees of rotation. This is much larger than the 0.002 degree rotation repeatability typically achieved by fixed-wavelength polarimeters. If the wavelength selector is a wheel or turret of discrete bandpass filters or if the wavelength is selected with a manually interchangeable bandpass filters, wavelength errors can also arise due to the temperature coefficient of the filters, the inclination of the filter to the beam path, or the degradation of the filter due to environmental conditions such as humidity or mechanical shock.
Accordingly, there is a need in the art to address the error that arises in polarimetry measurements.